1. Field of the invention
The present invention relates to a multi-phase outer-type permanent magnet PM stepping motor suitable for OA apparatus such as a printer, a high-speed facsimile equipment, a copying machine for normal paper, etc.
2. Description of the Related Art
Description will be made about a related-art outer-type PM stepping motor with reference to FIGS. 18A and 18B and FIG. 19.
FIG. 18A is a vertically sectional side view illustrating a configuration of a single-phase outer-type PM stepping motor 20 which is a first related-art example, and FIG. 18B is a partially cutaway perspective view of the same.
As shown in FIGS. 18A and 18B, this single-phase outer-type PM stepping motor 20 of the first related-art example is provided with a rotor 21 and a stator 23, and the rotor 21 has N (north) and S (south) magnetic poles formed alternately circumferentially on the inner circumferenitial surface.
Further, the stator 23 is disposed so that the outer circumferential surface of the stator 23 is opposite to the magnetic poles formed on the inner circumferential surface of the rotor 21 at a distance, and comb-like magnetic poles are formed in the stator 23.
In FIGS. 18A and 18E, the reference numeral 22 represents a rotating shaft; 24, a stator coil; 26, a motor mounting plate; and 27, a bearing.
FIG. 19 is a longitudinally sectional side view of a two-phase outer-type PM stepping motor 30 which is a second related-art example.
As shown in FIG. 19, this outer-type PM stepping motor 30 is also provided with a rotor 31 and a stator 33, and the stator 33 is constituted by first and second stator portions 33a and 33b stacked one on another in the axial direction.
In FIG. 19, the reference numeral 32 represents a rotating shaft; 34, a stator coil; 36, a motor mounting plate; and 37, a bearing.
The single-phase outer-type PM stepping motor of the first related-art example as mentioned above had only one stator. Therefore, in order to determine the rotating direction of the motor, magnetic permeance or phase between the pole teeth of the stator and the magnetic poles of the rotor was shifted, or a mechanical method was used. Accordingly, the related-art single-phase PM stepping motor was unsuitable for equipments while required high-speed rotation or high torque.
On tile other hand, according to the two-phase outer-type PM stepping motor of the second related-art example which was developed to solve these problems, torque and speed have been improved greatly, but the two-phase outer-type PM stepping motor has problems as follows.
(1) The number of lead wires for coils is large (i.e. at least four lead wires ), and it is necessary to use at least eight transistors in a driving circuit. PA0 (2) A torque ripple is so large that vibration is large. PA0 (3) When the step angle is made small, a large number of magnetic poles must be formed, causing a problem on machining. PA0 (4) It is difficult to obtain high torque with a fine step angle.
It is an object of the present invention to provide a multi-pilase outer-type PM stepping motor in which tile foregoing problems can be solved.